The invention relates generally to magnetic field generation mechanisms used in compact, thin magneto-optical recording/playback devices that record and erase by impressing a prescribed biased magnetic field on a magneto-optical disc. The magneto-optical disc may be made from a switched connection multilayer film capable of direct overwrite by two or more magnets that generate magnetic fields in different directions. Discs other than switched connection multilayer film magneto-optical discs may also be used.
In recent years, magneto-optical systems are implemented as external recording devices for computers. The magnetic field modulation system shown in FIGS. 24 and 25 is a known magneto-optical recording system in the prior art. This magnetic field modulation system writes data by heating a magneto-optical disc 001 to near the Curie temperature by spot irradiation of a laser beam from an optical head 002 and controlling the direction of the biased magnetic field impressed from a magnetic head 003.
Here, optical head 002 is disposed such that it can move in the radial direction of the magneto-optical disc, and can be moved to a prescribed position by the electromagnetic force applied by a magnet and a coil for optical head movement (not shown). A biased magnet 004 is supported by magnetic head 003 and can rotate freely within a prescribed angular range in the direction indicated by the arrows. A coil 005 for biased magnetic field is disposed so that it can rotate biased magnet 004 by the action of electromagnetic force. Therefore, by energizing coil 005 and thereby rotating biased magnet 004 to a certain angle, the direction of the biased magnetic field generated from biased magnet 004 can be controlled to the desired direction.
Data recording and erasing by this system of magnetic field inversion are performed as follows. First, in the case of erasing old data, the direction of the magnetic field generated from biased magnet 004 is set to a direction that denotes erasure. Laser beam is then continuously irradiated from optical head 002 on magneto-optical disc 001 so that it heats the disc to near the Curie temperature. Magneto-optical disc 001 heated to the Curie temperature looses its coercive force. When it cools, the disc is magnetized in the same direction as the biased magnetic field, i.e., a direction that denotes erasure.
Next, coil 005 is energized. Thus, electromagnetic force acts on biased magnet 004 and inverts it to a direction that denotes recording. The laser beam is then irradiated on magneto-optical disc 001 at the recording position so that it heats the disc near the Curie temperature. Magneto-optical disc 001 heated to the Curie temperature looses its coercive force. When it cools, the disc is magnetized in the same direction as the biased magnetic field, i.e., a direction that denotes recording.
In this way, in the magnetic field inversion system, magneto-optical disc 001 is rotated once to erase old data, and then magneto-optical disc 001 is rotated once again to write new data, thus requiring the magneto-optical disc to be rotated for a total of two times.
To solve this problem, systems referred to as direct overwrite have been actively researched recently. These systems erase old data while writing new data in one rotation of the magneto-optical disc. Two of these systems are known as the magnetic field modulation system and the light modulation system.
Of the light modulation systems, there is a system that uses a switched connection multilayer film. In the switched connection two-layer film system, which is the fundamental configuration of this type of system, a recording layer and an auxiliary layer are formed on the surface of the magneto-optical disc. Data are recorded and erased by applying an external initializing magnetic field and a recording magnetic field and controlling the strength of the laser beam irradiated on the recording layer. It is also known that by applying an inverted magnetic field whose direction is opposite to that of the recording magnetic field, data erasing can be facilitated.
The recording principle of the system that uses the switched connection two-layer film is illustrated in FIGS. 26 and 27. As shown in both figures, a recording layer 006 and an auxiliary layer 007 are formed on the magneto-optical disc. An initializing magnetic field 008, a recording magnetic field 009 and an inverted magnetic field 010 are impressed perpendicularly from outside as indicated by the arrows. Initialization magnetic field 008 and recording magnetic field 009 have the same direction. Inverted magnetic field 010 has an opposite direction. Also, initialization magnetic field 008 is larger than recording magnetic field 009.
The magnetic characteristics of recording layer 006 and those of auxiliary layer 007 vary depending on the temperature as shown in FIG. 28. At room temperature, the coercive force of recording layer 006 is higher than initialization magnetic field 008, while the coercive force of auxiliary layer 007 is lower than initialization magnetic field 008 and higher than recording magnetic field 009. Further, at temperature b of low-output laser heating, the coercive force of auxiliary layer 007 is lower than initialization magnetic field 008 and higher than recording magnetic field 009, while the coercive force of recording layer 006 is lower than recording magnetic field 009. In addition, at temperature c of high-output laser heating, the coercive forces of auxiliary layer 007 and recording layer 006 are lower than recording magnetic field 009. The direction of magnetization of auxiliary layer 007 inverts at temperature a between room temperature and the low-output laser heating temperature.
In performing recording and erasure, first, the magneto-optical disc is rotated while impressing initialization magnetic field 008 at room temperature. Since the coercive force of auxiliary layer 007 is lower than the initialization magnetic field 008 at room temperature, only auxiliary layer 007 is magnetized in one direction by initialization magnetic field 008. The direction of the magnetic field generated by initialization magnetic field 008 is a direction that denotes recording. Since the coercive force of recording layer 006 is higher than initialization magnetic field 008 at room temperature, it is not magnetized.
Next, as shown in FIG. 26, when a low-output laser beam 011 is irradiated on the magneto-optical disc at the position of the recording magnetic field, recording layer 006 looses its magnetic field because its coercive force is lower than the recording magnetic field 009 at heating temperature b caused by the low-output laser beam 011. Then, recording layer 006 is magnetized when it cools. However, since the switched connection force between recording layer 006 and auxiliary layer 007 is stronger than recording magnetic field 009, the direction of magnetization of recording layer 006 becomes opposite to that of recording magnetic field 009. In other words, the direction of magnetization of recording layer 006 is not the direction that denotes recording, but rather it is the opposite direction that denotes erasure. Since the direction of inverted magnetic field 010 is opposite to that of recording magnetic field 009, recording layer 006 is more reliably magnetized in the direction that denotes erasure. Auxiliary layer 007 is not magnetized since its coercive force is higher than recording magnetic field 009 at low-output heating temperature b.
Next, when a high-output laser beam 012 is irradiated on the magneto-optical disc at the position of the recording magnetic field as shown in FIG. 27, recording layer 006 and auxiliary layer 007 loose their respective magnetic fields since their coercive forces are lower than recording magnetic field 009 at heating temperature c caused by high-output laser beam 012. When the disc cools, recording layer 006 is magnetized in a direction that denotes recording by recording magnetic field 009.
When the respective magnetic characteristics of the recording layer and the auxiliary layer are considered in the above system that uses a switched connection multilayer film, the initialization magnetic field requires a magnetic field strength of 2.5 to 7 kilooersteds to invert the direction of magnetization of the auxiliary layer and avoid any effect on the recording layer. Also, the inverted magnetic field should have a magnetic field strength of 1 to 2 kilooersteds to erase any unerased data and avoid any effect on the auxiliary layer.
However, since the magneto-optical disc is generally housed inside a cartridge, a magnetic field generation device is required to impress a magnetic field frown outside the cartridge in order to obtain the prescribed magnetic field strength on the surface of the magneto-optical disc, which increases the size of the magneto-optical recording/playback device itself. Also, the effect of leakage magnetic field around the magneto-optical recording/playback device causes problems.
Therefore, in order to suppress the leakage magnetic field as much as possible and configure the magneto-optical recording/playback device as small as possible, the magnetic head must be thin and have a dimension that allows it to enter the shutter (which can be freely opened and closed) provided on the cartridge when it is opened. For example, in the case of a disc diameter of 3.5 inches and a magneto-optical recording/playback device of 1 inch high, the magnetic head must be less than 7 mm high and 20 mm wide.
The configuration in FIG. 29 can be considered as an example in which the magnetic head is kept within that kind of range and the thickness of the magnet in the magnetic head is maximized. As shown, a magneto-optical disc 013 is housed inside a cartridge (not shown) such that it can rotate freely. The disc is rotated at the prescribed rpm by a motor (not shown). The cartridge has opposing windows which are provided with shutters (not shown) that can slide freely. In the figure, an optical head 014 and a magnetic head 015 enter the cartridge through the windows of the cartridge and oppose each other with magneto-optical disc 013 between them. Magnets 016 for initialization magnetic field that generate magnetic fields perpendicular to magneto-optical disc 013 are mounted on the right and left sides of magnetic head 015. In order to maximize the magnetic field strength of magnets 016 for initialization magnetic field, they are made the same thickness as magnetic head 015.
The magnetic field distribution generated by the magnetic head on the magneto-optical disc is shown in FIG. 30 with respect to the magnetic head position. As shown in the figure, the magnetic field is generated with left and right symmetry. While the recording magnetic field is sufficiently strong at the center position of optical head 014, the initialization magnetic field and the inverted magnetic field cannot be obtained with sufficient magnetic field strength.
To increase the initialization magnetic field, the use of a magnet for optical head movement that moves the optical head by electromagnetic force can be considered. An example is shown in FIG. 31. As shown in the figure, yokes 018 and 020 are disposed parallel to each other on the left and right sides of optical head 014 in the direction of movement of optical head 014 (i.e., the direction perpendicular to the paper surface). Magnets 019 for optical head movement are mounted outside yokes 020. Magnets 019 generate a magnetic field in a direction parallel to magneto-optical disc 013 and in a direction from the inside to the outside. Also, a coil 017 for optical head movement, which surrounds inside yokes 018, is attached to optical head 014. Therefore, coil 017 interlinks with the magnetic flux generated by magnets 019 between yokes 018 and 019. By energizing coil 017 and causing electromagnetic force to act on coil 017 and optical head 014, it is possible to move optical head 014 to a prescribed position.
In this example, since the magnetic field generated by initialization magnets 016 of the magnetic head and the magnetic field generated by magnets 019 for optical head movement strengthen each other as shown in FIG. 32, a stronger magnetic field than in the case of FIG. 30 can be obtained. However, the initialization magnetic field and the inverted magnetic field still cannot be obtained with sufficient strength.
Further, the addition of a magnet 021 for inverted magnetic field as shown in FIG. 33 can be considered for increasing the inverted magnetic field. As shown, a magnet 022 for initialization magnetic field and magnet 021 for inverted magnetic field are mounted on magnetic head 015. The magnetic field of magnet 021 is in a direction parallel to magnetic field of magnet 022, but the direction of the magnetic field of magnet 022 points down in the figure, while the direction of the magnetic field of magnet 021 points up in the figure.
In this example, the magnetic field distribution is generated asymmetrically as shown in FIG. 34. A sufficient magnitude is obtained for the inverted magnetic field. However, a sufficient magnetic field strength is not obtained for the recording magnetic field and the initialization magnetic field.
As described above, since the thickness and the width of the magnets are restricted in order to make the magnetic head thin and compact, sufficient magnetic field strengths cannot be obtained for the initialization magnetic field and the inverted magnetic field.
The invention is intended to solve this problem. Its purpose is to offer a magnetic field generation mechanism that generates an initialization magnetic field and a recording magnetic field capable of direct overwrite and facilitates the realization of a compact magneto-optical recording/playback device.